The call for nominations for the 2019 EurAAP Leopold B. Felsen Award for Excellence in Electrodynamics is open.

The Award was originally established jointly by the University of Siena and the University of Sannio,
funded through a donation from Michael and Judy Felsen in fulfillment of the last wishes of their father,
Professor Leo Felsen (1924-2005). Since 2015, the Award is established by the
European Association on Antennas and Propagation (EurAAP).

The main purpose of the Award is to keep alive Prof. Felsen’s memory and scientific legacy,
as well as to foster academic excellence in the electromagnetics community,
by giving recognition to outstanding fundamental contributions from early stage researchers in electrodynamics.

The Award, by nomination only, will be presented annually, as a career award, to an early stage researcher.
Eligible nominees should be under 40 years of age at the submission deadline.
Previous awardees (also from the former Sannio and Siena editions) are not eligible.

The Award consists of a prize of 4,000 euro, which will be funded by the Felsen Family through a donation,
and will be officially presented at the banquet of the upcoming
[13th European Conference on Antennas and Propagation] (EuCAP 2019).

Candidates from all areas of Electromagnetics are eligible to apply.
Selection will be based on quality and significance of the candidates’ contributions to the field, and on the
recommendation of the referees.

Recent studies on fully dielectric multilayered metamaterials have shown that
the negligibly small nonlocal effects (spatial dispersion) typically observed
in the limit of deeply subwavelength layers may be significantly enhanced by
peculiar boundary effects occurring in certain critical parameter regimes.
These phenomena, observed so far in periodic and randomly disordered
geometries, are manifested as strong differences between the exact optical
response of finite-size metamaterial samples and the prediction from
conventional effective-theory-medium models based on mixing formulae. Here,
with specific focus on the Thue-Morse geometry, we make a first step toward
extending the studies above to the middle-ground of aperiodically ordered
multilayers, lying in between perfect periodicity and disorder. We show that,
also for these geometries, there exist critical parameter ranges that favor the
buildup of boundary effects leading to strong enhancement of the (otherwise
negligibly weak) nonlocality. However, the underlying mechanisms are
fundamentally different from those observed in the periodic case, and exhibit
typical footprints (e.g., fractal gaps, quasi-localized states) that are
distinctive of aperiodic order. The outcomes of our study indicate that
aperiodic order plays a key role in the buildup of the aforementioned boundary
effects, and may also find potential applications to optical sensors, absorbers
and lasers.

The recently proposed digital coding metasurfaces make it possible to control electromagnetic (EM) waves in real time, and allow the implementation of many different functionalities in a programmable way. However, current configurations are only space-encoded, and do not exploit the temporal dimension. Here, we propose a general theory of space-time modulated digital coding metasurfaces to obtain simultaneous manipulations of EM waves in both space and frequency domains, i.e., to control the propagation direction and harmonic power distribution simultaneously. As proof-of-principle application examples, we consider harmonic beam steering, beam shaping, and scattering-signature control. For validation, we realize a prototype controlled by a field-programmable gate array, which implements the harmonic beam steering via an optimized space-time coding sequence. Numerical and experimental results, in good agreement, demonstrate good performance of the proposed approach, with potential applications to diverse fields such as wireless communications, cognitive radars, adaptive beamforming, holographic imaging.

Nonlocal (spatial-dispersion) effects in multilayered metamaterials composed of periodic stacks of alternating, deeply subwavelength dielectric layers are known to be negligibly weak. Counterintuitively, under certain critical conditions, weak nonlocality may build up strong boundary effects that are not captured by conventional (local) effective-medium models based on simple mixing formulas. Here we show that this phenomenon can be fruitfully studied and understood in terms of error propagation in the iterated maps of the trace and antitrace of the optical transfer matrix of the multilayer. Our approach effectively parameterizes these peculiar effects via remarkably simple and insightful closed-form expressions, which enable direct identification of the critical parameters and regimes. We also show how these boundary effects can be captured by suitable nonlocal corrections.

Our paper on “Boundary effects of weak nonlocality in multilayered dielectric metamaterials,”
in collaboration with Andrea Alù (ASRC-CUNY),
has been accepted for publication in Physical Review Applied.

Coding metasurfaces, composed of only two types of elements arranged according to a binary code, are attracting a steadily increasing interest in many application scenarios. In this study, we apply this concept to attain diffuse scattering at THz frequencies. Building up on previously derived theoretical results, we carry out a suboptimal metasurface design based on a simple, deterministic and computationally inexpensive algorithm that can be applied to arbitrarily large structures. For experimental validation, we fabricate and characterize three prototypes working at 1 THz, which, in accordance with numerical predictions, exhibit significant reductions of the radar cross-section, with reasonably good frequency and angular stability. Besides the radar-signature control, our results may also find potentially interesting applications to diffusive imaging, computational imaging, and (scaled to optical wavelengths) photovoltaics.

Our paper on “Suboptimal coding metasurfaces for terahertz diffuse scattering,”
in collaboration with Tie Jun Cui’s Group (Southeast University),
has been accepted for publication in Scientific Reports.

The integration of structures supporting Bloch surface waves (BSWs) with optical fibers is highly desirable, since it would enable the development of high-figure-of-merit miniaturized all-fiber optrodes, opening new pathways within the “lab-on-fiber” roadmap. Here, the first experimental demonstration of grating-assisted excitation of BSWs on the tip of single-mode fibers in the near-infrared region is provided. This is attained via fabrication of a 1D diffraction grating on the fiber facet, and subsequent deposition of a 1D photonic crystal. In spite of a resonance broadening due to grating-induced morphological perturbations, the measured Q-factor of 50 is still higher than typical lab-on-tip plasmonic-probe benchmarks. With a view toward biomolecular sensing, a surface sensitivity of 1.22 nm nm−1 of homogeneous overlay deposited over the active region, which is in line with most plasmonic optrodes largely used in connection with optical fibers, is evaluated. The results also highlight the current limitations and the challenges to face for the development of advanced BSW-based fiber-tip platforms for biological sensing applications.

A Tutorial Session on Metamaterials was held at the 5th IEEE International Workshop on Metrology
for AeroSpace (MetroAeroSpace 2018) in Rome, Italy. Tutorial lectures were given by Profs.
Vincenzo Galdi (University of Sannio), Alessio Monti (Niccolò Cusano University),
and Mirko Barbuto (Niccolò Cusano University).

The paper “Roadmap on transformation optics” [1]
has been published in the Journal of Optics. Section 3 contains a
perspective on nonlocal and non-Hermitian extensions authored by Prof. Vincenzo Galdi.

Transformation optics asks, using Maxwell’s equations, what kind of electromagnetic medium recreates some smooth deformation of space? The guiding principle is Einstein’s principle of covariance: that any physical theory must take the same form in any coordinate system. This requirement fixes very precisely the required electromagnetic medium. The impact of this insight cannot be overestimated. Many practitioners were used to thinking that only a few analytic solutions to Maxwell’s equations existed, such as the monochromatic plane wave in a homogeneous, isotropic medium. At a stroke, transformation optics increases that landscape from ‘few’ to ‘infinity’, and to each of the infinitude of analytic solutions dreamt up by the researcher, there corresponds an electromagnetic medium capable of reproducing that solution precisely. The most striking example is the electromagnetic cloak, thought to be an unreachable dream of science fiction writers, but realised in the laboratory a few months after the papers proposing the possibility were published. But the practical challenges are considerable, requiring meta-media that are at once electrically and magnetically inhomogeneous and anisotropic. How far have we come since the first demonstrations over a decade ago? And what does the future hold? If the wizardry of perfect macroscopic optical invisibility still eludes us in practice, then what compromises still enable us to create interesting, useful, devices? While three-dimensional (3D) cloaking remains a significant technical challenge, much progress has been made in two dimensions. Carpet cloaking, wherein an object is hidden under a surface that appears optically flat, relaxes the constraints of extreme electromagnetic parameters. Surface wave cloaking guides sub-wavelength surface waves, making uneven surfaces appear flat. Two dimensions is also the setting in which conformal and complex coordinate transformations are realisable, and the possibilities in this restricted domain do not appear to have been exhausted yet. Beyond cloaking, the enhanced electromagnetic landscape provided by transformation optics has shown how fully analytic solutions can be found to a number of physical scenarios such as plasmonic systems used in electron energy loss spectroscopy and cathodoluminescence. Are there further fields to be enriched? A new twist to transformation optics was the extension to the spacetime domain. By applying transformations to spacetime, rather than just space, it was shown that events rather than objects could be hidden from view; transformation optics had provided a means of effectively redacting events from history. The hype quickly settled into serious nonlinear optical experiments that demonstrated the soundness of the idea, and it is now possible to consider the practical implications, particularly in optical signal processing, of having an ‘interrupt-without-interrupt’ facility that the so-called temporal cloak provides. Inevitable issues of dispersion in actual systems have only begun to be addressed. Now that time is included in the programme of transformation optics, it is natural to ask what role ideas from general relativity can play in shaping the future of transformation optics. Indeed, one of the earliest papers on transformation optics was provocatively titled ‘General Relativity in Electrical Engineering’. The answer that curvature does not enter directly into transformation optics merely encourages us to speculate on the role of transformation optics in defining laboratory analogues. Quite why Maxwell’s theory defines a ‘perfect’ transformation theory, while other areas of physics such as acoustics are not apparently quite so amenable, is a deep question whose precise, mathematical answer will help inform us of the extent to which similar ideas can be extended to other fields. The contributors to this Roadmap, who are all renowned practitioners or inventors of transformation optics, will give their perspectives into the field’s status and future development.

The call for nominations for the 2018 EurAAP Leopold B. Felsen Award for Excellence in Electrodynamics is open.

The Award was originally established jointly by the University of Siena and the University of Sannio,
funded through a donation from Michael and Judy Felsen in fulfillment of the last wishes of their father,
Professor Leo Felsen (1924-2005). Since 2015, the Award is established by the
European Association on Antennas and Propagation (EurAAP).

The main purpose of the Award is to keep alive Prof. Felsen’s memory and scientific legacy,
as well as to foster academic excellence in the electromagnetics community,
by giving recognition to outstanding fundamental contributions from early stage researchers in electrodynamics.

The Award, by nomination only, will be presented annually, as a career award, to an early stage researcher.
Eligible nominees should be under 40 years of age at the submission deadline.
Previous awardees (also from the former Sannio and Siena editions) are not eligible.

Candidates from all areas of Electromagnetics are eligible to apply.
Selection will be based on quality and significance of the candidates’ contributions to the field, and on the
recommendation of the referees.

The XXXV Edition of the EUPROMETA Distributed Doctoral School on Metamaterials will be held
in Rome, Italy, from December 18-22, 2017. The school will be focused on Advanced Electromagnetic Materials and Surfaces for Novel Wave Phenomena.

Our paper on “Enhancement and interplay of first- and second-order spatial
dispersion in metamaterials with moderate-permittivity inclusions” [1]
in collaboration with Carlo Rizza (University of l’Aquila) and Alessandro Ciattoni (CNR-SPIN),
has been published in Physical Review B as a Rapid Communication.

We investigate a class of multilayered metamaterials characterized by moderate-permittivity inclusions and low average permittivity. Via first-principles calculations, we show that in such a scenario, first- and second-order spatial dispersions may exhibit a dramatic and nonresonant enhancement, and may become comparable with the local response. Their interplay gives access to a wealth of dispersion regimes encompassing additional extraordinary waves and topological phase transitions. In particular, we identify a configuration featuring bound and disconnected isofrequency contours. Since they do not rely on high-permittivity inclusions, our proposed metamaterials may constitute an attractive and technologically viable platform for engineering nonlocal effects in the optical range.

Our paper on “Enhancement and interplay of first- and second-order spatial
dispersion in metamaterials with moderate-permittivity inclusions,”
in collaboration with Carlo Rizza (University of l’Aquila) and Alessandro Ciattoni (CNR-SPIN),
has been accepted for publication as a Rapid Communication in Physical Review B.

Coding metasurfaces, based on the combination of two basic unit cells with out-of-phase responses, have been the subject of many recent studies aimed at achieving diffuse scattering, with potential applications to diverse fields ranging from radar-signature control to computational imaging. Here, via a theoretical study of the relevant scaling-laws, the physical mechanism underlying the scattering-signature reduction is elucidated, and some absolute and realistic bounds are analytically derived. Moreover, a simple, deterministic suboptimal design strategy is introduced that yields results comparable with those typically obtained by approaches based on brute-force numerical optimization, at a negligible fraction of their computational burden, thereby paving the way to the design of structures with arbitrarily large electrical size. Results are corroborated by rigorous full-wave numerical simulations and microwave experiments, and may be of interest in a variety of application fields, such as the design of low-scattering targets and illumination apertures for computational imaging, not necessarily restricted to electromagnetic scenarios.

Our paper on “Transformation-optics-based design of a metamaterial radome for extending the scanning
angle of a phased array antenna” [1],
in collaboration with MBDA,
has been published (early view) in the IEEE Journal of Multiscale and Multiphysics Computational Techniques.

We apply the transformation-optics approach to the design of a metamaterial radome that can extend the scanning angle of a phased-array antenna. For moderate enhancement of the scanning angle, via suitable parameterization and optimization of the coordinate transformation, we obtain a design that admits a technologically viable, robust and potentially broadband implementation in terms of thin-metallic-plate inclusions. Our results, validated via finite-element-based numerical simulations, indicate an alternative route to the design of metamaterial radomes which does not require negative-valued and/or extreme constitutive parameters.

Our paper on “Exceptional points of degeneracy and PT-symmetry in photonic coupled chains of scatterers”
[1]
in collaboration with Filippo Capolino’s Group (University of California at Irvine),
has been accepted for publication in Physical Review B.

We demonstrate the existence of exceptional points of degeneracy (EPDs) of periodic eigenstates in non-Hermitian coupled chains of dipolar scatterers. Guided modes supported by these structures can exhibit an EPD in their dispersion diagram at which two or more Bloch eigenstates coalesce, in both their eigenvectors and eigenvalues. We show the emergence of a second-order modal EPD associated with the parity-time (PT) symmetry condition, at which each particle pair in the double chain exhibits balanced gain and loss. Furthermore, we also demonstrate a fourth-order EPD occurring at the band edge. Such a degeneracy condition was previously referred to as a degenerate band edge in lossless anisotropic photonic crystals. Here, we rigorously show it under the occurrence of gain and loss balance for a discrete guiding system. We identify a more general regime of gain and loss balance showing that PT symmetry is not necessary to attain EPDs. Moreover, we investigate the degree of detuning of the EPD when the geometrical symmetry or balanced condition is broken. Furthermore, we demonstrate a realistic implementation of the EPD in a coupled chain made of pairs of plasmonic nanospheres and active core-shell nanospheres at optical frequencies. These findings open avenues toward superior light localization and transport with application to high-Q resonators utilized in sensors, filters, low-threshold switching and lasing.

We report on the first demonstration of a proof-of-principle optical fiber ‘meta-tip’, which integrates a phase-gradient plasmonic metasurface on the fiber tip. For illustration and validation purposes, we present numerical and experimental results pertaining to various prototypes implementing generalized forms of the Snell’s transmission/reflection laws at near-infrared wavelengths. In particular, we demonstrate several examples of beam steering and coupling with surface waves, in fairly good agreement with theory. Our results constitute a first step toward the integration of unprecedented (metasurface-enabled) light-manipulation capabilities in optical-fiber technology. By further enriching the emergent ‘lab-on-fiber’ framework, this may pave the way for the widespread diffusion of optical metasurfaces in real-world applications to communications, signal processing, imaging and sensing.

Our paper on “Magnified imaging based on non-Hermitian nonlocal cylindrical
metasurfaces” [1],
in collaboration with Andrea Alù’s Group (University of Texas at Austin),
has been published in Physical Review B.

We show that a cylindrical lensing system composed of two metasurfaces with suitably tailored non-Hermitian (i.e., with distributed gain and loss) and nonlocal (i.e., spatially dispersive) properties can perform magnified imaging with reduced aberrations. More specifically, we analytically derive the idealized surface-impedance values that are required for “perfect” magnification and imaging and elucidate the role and implications of non-Hermiticity and nonlocality in terms of spatial resolution and practical implementation. For a basic demonstration, we explore some proof-of-principle quasilocal and multilayered implementations and independently validate the outcomes via full-wave numerical simulations. We also show that the metasurface frequency-dispersion laws can be chosen so as to ensure unconditional stability with respect to arbitrary temporal excitations. These results, which extend previous studies on planar configurations, may open intriguing venues in the design of metastructures for field imaging and processing.

Our paper on “Magnified imaging based on non-Hermitian nonlocal cylindrical
metasurfaces,”
in collaboration with Andrea Alù’s Group (University of Texas at Austin),
has been accepted for publication in Physical Review B.

Our paper on “Exceptional points of degeneracy and PT-symmetry in photonic coupled chains of scatterers,”
in collaboration with Filippo Capolino’s Group (University of California at Irvine),
has been accepted for publication in Physical Review B.